1. Identify the physical situation

We have a mixture of two liquids:

• isohexane
• 3-methylpentane

Their boiling points are very close:

• one boils at 60^\mathrm{C}
• the other boils at 63^\mathrm{C}

2. Choose the correct separation method

When the difference in boiling points is small, simple distillation is not efficient.

For liquids having close boiling points, the correct method is fractional distillation.

So the separation method must be: $\text{fractional distillation}$

3. Determine which liquid distils first

In distillation, the liquid with the lower boiling point vaporizes and comes out first.

Between 60^\mathrm{C} and 63^\mathrm{C}, the lower boiling point is: 60^\mathrm{C}

Now, among the two compounds:

• 3-methylpentane is more branched than isohexane.
• Greater branching generally lowers boiling point because it decreases surface area and weakens intermolecular van der Waals forces.

Thus: $\text{3-methylpentane has the lower boiling point} \Rightarrow \text{it distils first}$

4. Match with options

• A: fractional distillation, 3-methylpentane
• B: simple distillation, 3-methylpentane
• C: fractional distillation, isohexane
• D: simple distillation, isohexane

Hence the correct option is: $\boxed{\text{A}}$

5. Comparison with stored correct answer

Stored correct answer = C

But from boiling point logic and branching effect, the first fraction should be 3-methylpentane, not isohexane.

Therefore, I disagree with the stored answer.